Chemical insecticides are an integral component of modern agriculture, and are an effective means for reducing crop damage by controlling insect pests. However, chemical agents are under continuous scrutiny due to the potential for environmental contamination, selection of resistant populations of agronomic pests, and toxicity to non-target organisms such as beneficial insects, aquatic organisms, animals and humans. As a result, alternative strategies for insect control are being sought that are effective and yet benign to non-target populations and the environment. One of these strategies is to utilize the mechanisms of naturally occurring pathogens of target pest populations.
Examples of this type of strategy include the use of various forms of the δ-endotoxin produced by the soil dwelling microorganism Bacillus thuringiensis (Bt) as pesticidal agents. These polypeptides have been found to be specifically toxic to particular insects, and microbial formulations have been used commercially for many years as foliarly applied insecticides. It has also recently been found that various forms of the Bt toxin can be toxic to insect pests when expressed inside the tissues of plants on which the insects feed.
Many arthropods express polypeptides capable of killing or incapacitating various pests. Therefore arthropods have been identified as a group of organisms producing polypeptides possessing pesticidal properties. In fact, scorpion venom contains insect-selective toxins affecting ion channels (Zlotkin et al. (1985) Arch. Biochem. Biophys. 240:877-87). The toxicity of the different polypeptides operates through multiple pharmacological and biochemical pathways (Zlotkin et al. (1971) Biochimie (Paris), 53:1073-1078).
Transgenic dicot plants expressing a toxin obtained from Androctonus australis scorpions have been created (U.S. Pat. No. 5,177,308). The AaIT expressing plants have been crossbred to transgenic plants carrying Bt δ-endotoxin yielding plants with two independent insect-specific toxin traits (U.S. Pat. No. 5,177,308).
Insects predominate as rice pests throughout Asia and are most serious in tropical regions where over 60 species are pests. The most serious rice insect pests are commonly categorized as Homoptera (sucking pests such as leafhoppers and planthoppers) and Lepidoptera (stem borers and defoliators).
The identification of new pesticidal polypeptides is desirable for use in pest-management strategies. It is of particular importance to identify pesticidal toxins that are active against insect pests from the orders Homoptera and Lepidoptera, and against insects that have developed resistance to Bt toxins.